.todos{
    &__create-input {
        margin: 0 auto 5px;
        width: 300px;
    }
    &__create-button {
        display: block;
        margin: 0 auto 20px;
    }
    &__task{
        &--completed {
            text-decoration: line-through;
        }
    }
}
/*
.todos{
    &__create-input {

    }
}
等价于
.todos__create-input{

}
*/
